Steam turbines are generally large, heavy steel machines containing a rotor having a plurality of blades against which high velocity steam is directed to cause the rotor to turn. Normally an electric generator is affixed to the shaft of the rotor so as to produce electricity. Frequently the steam passes serially through several sets of rotor blades, each set being a "stage" of the turbine designed to make most efficient use of the steam it receives. The steam generally enters the turbine to the first stage at a high pressure and temperature, and passes through successive stages at lower pressure and lower temperature as it passes each stage. These turbines are individually designed, and hand made. The parts that become worn over usage are the rotor, its blades and bearings, and the nozzle portions which direct the steam against the rotor blades. Erosion occurs at the nozzle area and at the rotor blades, with corresponding losses in efficiency until it is necessary to replace the eroded parts. Generally in the nozzle area there are vanes which direct the steam in a precisely predetermined direction which is deemed to be most efficient with respect to the blades of the rotor. Usually the vanes are of a teardrop or airfoil shape with a large rounded leading edge and a short thin trailing edge. These vanes individually, or as a segment of several adjacent vanes, are welded into the terminus of the steam passageway. Removal of eroded vanes and replacement by new vanes is a time-consuming and expensive operation, as is readily appreciated.
The prior art nozzle and diaphragm assembly construction, widely used throughout the industry, is made up from a series of individually assembled component parts into sub-assembly weld fabrications. These sub-assemblies are then welded together to produce a nozzle and/or diaphragm assembly. After each sub-assembly and main weld fabrication a stress relief is required. Each assembly must be subjected to a furnace stress relief for approximately eight hours at 1300.degree. F. to 1400.degree. F. Unfortunately, it is not possible or practical to determine the overall effects these stress reliefs have on the most critical part of the steam path assembly--"The Nozzle Partition" without destroying the part. It has been regularly observed during a unit inspection, after approximately three years of operation from the time of initial startup, that the nozzle partitions have suffered extensive fatigue failures. These results will vary from unit to unit without any boiler degradation. The problem is primarily due to a reduction in the mechanical properties and fatigue strength of the nozzle partitions. The reduced life expectancy results are directly attributed to and effected by the number of actual stress reliefs to which the nozzle partitions are subjected and the thermal transient control of the process. This is one of the major problems that is overcome by the present invention.
A general object of this invention is the provision of a turbine having readily replaceable vane portions including nozzle or diaphragm portions. A specific object of this invention is to provide a novel retrofit for steam turbine in which replaceable nozzle portions are provided. It is another object of the invention to provide a novel retrofit steam turbine that has nozzle or diaphragm portions that are not welded into the turbine housing. Still other objects will become apparent from the more detailed description which follows.